The last decade has seen a dramatic increase in the use of polymer membranes as effective, economical and flexible tools for many gas separations. The processability, gas solubility, and selectivity of several classes of polymers (such as polyimides, polysulfones, polyesters, and the like) have led to their use in a number of successful gas separation applications. A drawback to the use of polymer membranes for gas separation can be their low permeability or inadequate selectivity. In most instances, the success of a given membrane rests on achieving an appropriate combination of adequate permeability and selectivity.
Polymer membranes can be used for air separation, for the recovery of hydrogen from mixtures of nitrogen, carbon monoxide and methane, and for the removal of carbon dioxide from natural gas. For these applications, glassy polymer membranes provide high fluxes and excellent selectivities based on size differences of the gas molecules being separated.
Separation of hydrogen (H2) and carbon dioxide (CO2) from mixed gas streams is of major industrial interest. Current separation technologies require cooling of the process gas to ambient temperatures. Significant economic benefit could be realized if these separations are performed at elevated temperatures (greater than 150° C.). Consequently, much effort is directed at identifying and developing polymers that are chemically and mechanically stable at elevated temperatures and high pressures. Linear polybenzimidazole is an example of such a polymer. Representative patents and papers that describe membranes of linear polybenzimidazole include U.S. Pat. No. 2,895,948 to K. C. Brinker et al. entitled “Polybenzimidazoles,” which issued Jul. 21, 1959; RE 26,065 entitled “Polybenzimidazoles and Their Preparation,” which reissued to C. S. Marvel et al. on Jul. 19, 1966; “Polybenzimidazoles, New Thermally Stable Polymers,” H. Vogel et al., J. Poly. Sci., vol. L., pp. 511–539, 1961; “Polybenzimidazoles II,” H. Vogel et al., J. Poly. Sci. Part A, vol. 1, pp. 1531–1541, 1963; U.S. Pat. No. 3,699,038 to A. A. Boom entitled “Production of Improved Semipermeable Polybenzimidazole Membranes, which issued Oct. 17, 1972; U.S. Pat. No. 3,720,607 to W. C. Brinegar entitled “Reverse Osmosis Process Employing Polybenzimidazole Membranes,” which issued Mar. 13, 1973; U.S. Pat. No. 3,737,042 entitled “Production of Improved Semipermeable Polybenzimidazole Membranes,” which issued to W. C. Brinegar on Jun. 5, 1973; and U.S. Pat. No. 4,933,083 entitled “Polybenzimidazole Thin Film Composite Membranes,” which issued to R. Sidney Jones Jr. on Jun. 12, 1990, all of which are incorporated by reference herein. These patents and papers show that, for years, polybenzimidazole membranes have been useful for liquid phase separations such as reverse osmosis separations, ion exchange separations, and ultrafiltration.
Polybenzimidazole is also useful for gas separations. In U.S. Pat. No. 6,681,648 to Robert C. Dye et al. entitled “Meniscus Membranes for Separations,” for example, meniscus-shaped polybenzimidazole supported on a stainless steel substrate was useful for separating H2 from an H2/CO2 mixture, and CO2 from a CO2/CH4 mixture, and that membrane performance improves as the temperature increases from 25° C. to 250° C.
The mechanical properties of polybenzimidazole may be improved by cross-linking (see, for example, U.S. Pat. No. 4,020,142 to Howard J. Davis et al. entitled “Chemical Modification of Polybenzimidazole Semipermeable Membranes,” which issued Apr. 26, 1977). According to the '142 patent, crosslinked polybenzimidazole is tougher than non-cross-linked analogs and shows improved compaction resistance during prolonged usage at higher pressures. While cross-linked polybenzimidazole has been shown to be useful for liquid separations (separations in acid waste streams, reverse osmosis separations, ion exchange separations, and ultrafiltration separations), there is little information available related to gas separation using cross-linked polybenzimidazole.
Accordingly, an object of the present invention is to provide a method for separating gases using cross-linked polybenzimidazole.
Another object of the invention is to provide a cross-linked polybenzimidazole membrane for gas separation.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.